In the field of clinical tests, insoluble carrier particles are widely used for immunoassays, which comprise determination of antigens or antibodies in the samples. However, there is a problem that the reproducible values to be determined are not obtained when using a reagent for an immunoassay comprising insoluble carrier particles having been stored for a long time. Thus it is impossible to diagram a calibration curve with the same sensitivity, which results in an error of measurement.